Help with an MV45(a?)

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4thtimeaudiophile
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Help with an MV45(a?)

Post by 4thtimeaudiophile » Sun Jan 08, 2017 7:48 pm

I recently acquired an MV45 from a friend. It had been in storage for quite some time. I removed the top cover and cleaned the dust from the chassis and tubes. I assumed that the amp just needed to be powered up for awhile before being used. I connected a pair of Polk bookshelf speakers and nothing to the input. It powered up ok, but I noticed a slight hum in the left channel. Then, there was some crackling coming from that channel and the bias adjust LED for one of the tubes was flickering. I surmised that the bias adjust pot may have needed some cleaning. I sprayed some cleaner on both pots for the left channel. I went through the procedure for adjusting the bias on each of the EL34's. After waiting, I powered the amp back up. After about five minutes, there was smoke coming out of the chassis. I knew what that was. The first thought that crossed my mind was, "just lost an electrolytic cap". After the smoke cleared, I opened the bottom and my suspicion was confirmed. I removed the cap and confirmed that it was gone, along with some other physical evidence on the underside corner of the chassis.
I jury rigged an appropriate value of replacement capacitance in its place, but it is evident that there is damage beyond that. Right now, I am guessing that I lost my negative grid bias on one half of the push pull in the left channel. That half shows an LED bias light that will not go out. It also seems that because of no grid bias, the EL34 starts to go into runaway. I do not leave the power on long enough to let the plate glow red, but that EL34 gets warm right away while the others are almost cold.
I may have left out some pertinent details. I have been getting the schematics from this website, so it is getting a bit easier to approach this. I am at the point where I may have to get at the top side of the printed circuit board that is on the underside of the chassis. I certainly would appreciate any ideas and/or getting pointed in the right direction.

4thtimeaudiophile
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Re: Help with an MV45(a?)

Post by 4thtimeaudiophile » Wed Jan 11, 2017 11:03 pm

I have resolved the problem. There were quite a few runs on the circuit board that were cut, as part of the factory update, I assume. There was one which had flashed over, so I had assumed that run was burnt instead of cut. I mistakenly bridged it with a jumper wire. When I cut the jumper out, the problem went away. There were other minor issues, which I attribute to age. I replaced the smoked power supply electrolytic cap. The RCA input jacks on the back were quite intermittent, so those also were replaced. The latching main power switch lost the plastic protrusion. It has to be turned on and off with a screwdriver or pencil. The operation of the switch is fine. I touched up the bias controls for the EL34's per the instructions. This is a really sweet sounding amp, even with my average Polk Audio bookshelf speakers.

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Re: Help with an MV45(a?)

Post by admin » Wed Jan 11, 2017 11:05 pm

Congrats on the fix, I'm glad it working and giving you enjoyment!
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Big Dog RJ
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Re: Help with an MV45(a?)

Post by Big Dog RJ » Wed Jan 11, 2017 11:54 pm

Nice one mate!

Now that's what I call a classic tech with one classic amp! Wish there were more like you mate.

Back in home town, whenever we had problems with CJ gear, it was quite hard to find a reliable techie, hence could not ship defective units all the way to the US (12,000 miles across). Finally we found a good reliable tech, and he was a gem! Sadly passed away and still to this day there is no one like him. Some of the younger ones are learning quickly but they don't have the patience with tube electronics and electrostats for that matter...

Enjoy mate, the MV45 is definitely one sweet sounding amp indeed!
Cheers, RJ

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Re: Help with an MV45(a?)

Post by 4thtimeaudiophile » Sun Jan 15, 2017 8:58 pm

RJ,
Thanks for the compliment. As a young lad, I cut my teeth on vacuum tubes. Now, almost 50 years later, I am trying rekindle this old friendship. In 11th grade in high school, my entire term of electronics class was on vacuum tube and their circuits. I have not worked with them too extensively ever since then. One of the advantages of valve amplifiers is their relative simplicity compared with most of today's modern electronics. However, the solutions can often become elusive, in spite of simplicity. Intermittent problems are the worst. It is frustrating when a favorite piece of audio gear goes south on you. If something has given you many years of enjoyment, we can accept the reality of component failure. We can even bite the bullet to pay for the labor of a competent tech. Most consumer gear techs do audio and video setup and very little component level troubleshooting.

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